1. Field of the Invention
The present invention relates to a method for estimating carrier frequency offset introduced on an RF multicarrier signal received via a transmission channel on a direct downconversion analog receiver. The invention further relates to a method for compensating carrier frequency offset.
2. Description of the Related Technology
4G cellular wireless systems candidates like multi-carrier CDMA (MC-CDMA) or multi-carrier block spread CDMA (MCBS-CDMA) or single-carrier CDMA (SC-CDMA, also named cyclic prefix CDMA) and single-carrier block spread (SCBS-CDMA) have a high susceptibility to synchronization errors like carrier frequency offset (CFO) and IQ imbalance. These systems are multi-user systems and suffer from multi-user interference (MUI). The different synchronization errors of individual users all contribute to the total MUI, while the joint linear detectors used to separate the users are not built to remove the MUI caused by synchronization errors. If not properly estimated and compensated synchronization errors like carrier frequency offset (CFO) and IQ-imbalance cause a substantial performance degradation. The susceptibility to CFO is most prominent in the case of a multi-user up-link. Detecting the CFO's from different users in a multi-user up-link is only possible after the users are separated by the joint detector. Therefore the CFO is pre-compensated at each transmitting user terminal. If the CFO was not precisely measured and pre-compensated at the user terminals, the remaining CFO of the different users will cause MUI reducing the effectiveness of the joint detector at the base-station.
CDMA systems, due to the spreading of data, can operate in a wide range of Signal-to-Noise Ratios (SNRs), ranging from 0 dB till 30 dB. To obtain a useful accuracy at 0 dB a long preamble sequence, long in comparison to OFDM WLAN systems, is needed to average out the noise. Long preambles cause overhead, therefore a general strategy is adopted to measure the CFO and IQ-imbalance during an acquisition frame broadcasted by the base-station. Each user terminal measures its own receiver's CFO offset and IQ-imbalance. Because CFO and IQ-imbalance are slowly varying parameters subject to temperature drift, the measurements made on the acquisition frame can be used for a series of subsequent data bursts. At the terminal the CFO offset and IQ-imbalance are compensated for each down-link data burst, for an up-link data burst, the terminal pre-compensates for the measured CFO offset by artificially imposing the measured CFO on the data. Typically the base-station will have a high performant super-heterodyne receiver, so no IQ-imbalance compensation is necessary there. If not, the IQ-imbalance must be measured during user-terminal up-links.
Beside the obtained reduction in MUI, making a precise estimate of the CFO in the acquisition phase, has a supplemental advantage: Residual CFO must be tracked and this is usually done by inserting known pilots symbols in the data on which a phase rotation can be measured. If during the acquisition the CFO is precisely estimated, the remaining CFO will be small and the interval between the pilots can be large, reducing the pilot overhead.
Many known methods for estimating the CFO have the disadvantage that IQ imbalance is not taken into account, whereas for higher SNRs the presence of IQ-imbalance, when disregarded, impairs the CFO estimate.
A number of methods for CFO estimation taking into account the presence of IQ-imbalance already exist. In Feng Yan, Wei-Ping Zhu, and M. Omair Ahmad, “Carrier frequency offset estimation for ofdm systems with i/q imbalance,” Proc. the 47th IEEE Midwest Symposium on Circuits and Systems, MWSCAS '04., vol. 2, pp. 633-636, July 2004, an interesting time domain method is proposed that is based on the estimation of a cos φ, with φ the phase rotation caused by the CFO over a given time period. It is shown that this method performs well in comparison to order algorithms like S. Fouladifard and H. Shafiee, “A new technique for estimation and compensation of iq imbalance in ofdm receivers,” Proc. of the IEEE 8th International Conference on Communication Systems, vol. 1, pp. 224-228, November 2002, and Guanbin Xing, Manyuan Shen, and Hui Liu, “Frequency offset and i/q imbalance compensation for direct-conversion receivers,” Proc. (ICASSP '03) Acoustics, Speech, and Signal Processing, vol. 4, pp. 704-711, April 2003.
The method proposed by Feng Yan, Wei-Ping Zhu, and M. Omair Ahmad however has the disadvantage that it does not produce accurate results, for small CFO's under noisy conditions.